Modulating system



H. C. RESSLER MODULATING SYSTEM Filed Sept. 14, 1940 March 9, 1943.

2 Sheets-Sheet 1 I INVENTOR. jzgh RessZe' March 9, 1943. H C; RESSLER 2,313,583

MODULATING sYs hlnununun" wunlvunmununnq l UHLWUMuHl 1/ y? WHHHHHHHU o L y? www ui l "r w Y g nnnn M www w um ummm J ATTORNEY Patented Mar. 9, 1943 MODULATING, SYSTEM Hugh C. Ressler, West Brighton, N. Y., assigner to Faximile, Inc., New York, N. Y., a. corporation of Delaware Application september 14,1940, serial No. 356,751

Claims.

The present invention concerns electrical signal modulating systems and in particular balanced sub-carrier modulators.

One object -of the present invention is to provide an improved method of, and means for modulating an alternating current signal.

Another object is to provide improved modulation of a sub-carrier tone in accordance with the output of a photo-electric cell.

A further object is to provide an improved balanced modulation system for alternating current signals.

A still further object is to improve the stability of balance in a balanced modulating system.

Another object is to provide a stable modulator which may be balanced at any predetermined point, as, for instance, at maximum, minimum or intermediate values of light falling on a photoelectric cell providing the source of modulation.

These and other objects will be apparent from the detailed description of the figure of thedrawings.

One ofthe most satisfactory waysof transforming density variations of a picture or other facsimile subject matter into usable electrical signals is to scan the subject with a spot of light picking up the reflected light in a photo-electric cell and to utilize the photo-electric cell to modulate an alternating` current having afrequency in the range of from 2,000 to 20,000 cycles. This alternating current is called a sub-carrier and after modulation it may be transmitted to a receiving and reproducing point over a wire line or after a further modulation of a radio frequency carrier it may be transmitted thru the air.

The modulation of the sub-carrier by means of the photo-electric cell has, hitherto, been considered to a certain extent unsatisfactory. The

usual photo-electric cell is electrically equivalent to a very high resistance shunted by a small capacity. The variations in light from the subject falling on the yphoto-electric cell cause variation in this resistance. At the sub=carrier frequencies, however` the impedance of the cell capacity may heavily shunt the resistance and mask the desired changes in resistance. In order to overcome this dlmculty, circuits have been devised for balancing the photo-electric cell capacity and a certain portion of its resistance. While this mode of operation is possible, it is critical and tends to be unstable, requiring a good deal of attention.

modulator in which the adjustments 'may be easily made and which is quite stable in operation. The output of the photo-electric cell is applied to the grid of a tube, causing variations in its plate resistance. The steady component of plate resistance is balanced by the controlled plate resistance of a second and similar tube. The sub-carrier to be, modulated is fed into 'a balanced rectifier circuit and the modulation accomplished by unbalance between the two above mentioned tubes. Balancemay be produced for white or black subi ect matter or at any intermediate point. The balancing adjustment is easily made and the system operates in 'a stable manner, requiring little attention.

In the drawings is shown a particular embodiment cf the present invention, with curves to ex- 'plain its mode of operation.

Figure l shows a source of sub-carrier tone connected to primary 2 of the modulator input transformer 2-3-4. Secondary 4 is connected to primary l0 of the modulator output transformer |0|2|3 thru rectlflers '6 and 9. A third rectifier 1 is connected across the circuit between secondary 4 and primary l0 and a fourth rectier 8' is connected across in the opposite direction. The .combination of rectiers 6, 9', 1 and 8 may be regarded as the four arms of a symmetrical lattice composed of resistors whose values may be Changed by the voltage impressed across each resistor. Secondary i3 is connected to the utilization means I4 which may be a wire line, amplifier or other device for utilizing the modulated signals.

The modulation of sub-carrier signals is accomplished by means of tubes il! and i8 connected to mid-tap 5 on secondary d and midtap I l on primary l0 respectively. Tube ll has at least a cathode 2l heated by conventional"'means not shown, control grid 20and plate lilxPlate l@ is loaded by and receivescurrent thru' resistor l5 connected to battery 33. Plate l0 is also connected to midwpoint 5 of secondary 0. Grid 20 is connected to cathode 25 of photo# electric cell 25. Cell 25 receives light from lamp Bil-3l passing thru lenses 28 and 29 and modulated by subject matter on the surface of 'drum 32. Anode 2l of cell 25 is energized `from tap on battery 33% thru vdecoupling resistor di.'

Anode 2l is by-passed by condenser 00. Tube i0 includes at least cathode 2d, heated by conventional means not shown, control grid 23, 'and plate 22. Plate 22 is loaded by and receives plate current thru resistor lpreferably substantially The present invention concerns a balanced* 55 equal to resistor l5. Plate 22 is also connected to tap II on primary III. Control grid 23 is connected to variable tap 39 on potentiometer 38 which is bridged between points 36 and 31 on battery 33. Variation of point 39 alongreslstor 38 varies the balance thru -the modulator. Cathodes 2| and 24 are connected to point 34 on battery 33.

In operation, the bridge formed by rectiers 6, l, 8 and 9 is balanced so that no sub-carrier voltage is transferred from coil 4 to coil i0 when the instantaneous potential of mld-point 5 is equal to the instantaneous potential of mid-point II. Whenever-point II has an instantaneous potential greater than point 5 current of one phase iiows from coil 4 to coil I0 thru rectiiiers I- and 8 and Whenever theinstantaneous potential of point II is below the potentialof point 5, sub-carrier current flows from coil 4 to coil I0 thru rectifier 6 and 9 in reversed phase. Thus, the magnitude and phase of sub-carrier signals passed thru the bridge rectifier are determined by the relative instantaneous potentials of points 5 and II. The potential of point II may be adjusted by varying point 39 on potentiometer 38,

controlling the bias on grid 23. The potential of point 5 varies in accordance with the light falling on photocell 25. The potential of point II may be made equal to point 5 for any particular light value on photocell 25 from Zero toits maximum value. Thus, balance may be produced for maximum light and sub-carrier signal v passed thru the bridge in proportion to the decrease in light from maximum due to the reectioncoeiiicient of the subject on drum 32 or the balance may be made at zero light on photocell 23 and sub-carrier signals passed in proportion to the increase in light from zero. Many other balance conditions may be utilized, such as a Abalance at some predetermined light value, ora balance which passes a predetermined sub-carrier signal at zero light or at full light.

Rectifiers 6, I, 8 and 9 should be so chosen that they will operate in a satisfactory manner in a given system. While not intended to limit the invention, it has been found that copper-oxide rectiers having relatively small dimensions perate well -to modulate a sub-carrier of 3000 cycles from a high speed facsimile pick-up. The circuit is useful, however, over a Wide range of sub-carrier frequencies and with sources of modulating yoltage other than the photo-electric cell shown.

This system has many advantages over prior devices. As described above, the balance point may be adjusted in any desired manner. 'I'he adjustment of balance is easily made or changed at will. By making tubes I'I and I8 similar, the stability of balance is very great, due to the fact u that changes in supply voltages, ambient teniperature, etc., affect the system symmetrically. It will be evident that the system may be operated with tube II connected to point II and tube I8 connected to point 5.

The current generated in photoelectric cell25 in scanning a picture range in frequency from zero or direct current to an upper limit deteru mined by the speed and resolution of the scanning system. Large areas of white in the scanned copy produce substantially steady photoelectric currents while large areas of black produce substantially no current for relatively long periods of time. It is important that the phase -or sense of modulation be properly determined, so that therecorded copy will have a predetermined sense with respect tothe original copy. If the recording mechanism is such that a mark ing at the receiver from a blue print at the transmitter. The present system may be adjusted by means of potentiometer point 39 to produce full modulation for any value of light, on the photoelectric cell whichI permits positive or negative transmission and the predetermination of the white level.

Because of the very low frequency components of the photoelectric currents it. is essential that the picture frequency components be suppressed either at the modulator or by means of filter circuits connected after the modulator. Such filter circuits are expensive and complicated, so that one of the great advantages of the present system lies in the fact that picture frequency components are suppressed in the modulator. At balance, the currents in the two halves of coil I0 areequal and in opposite directions and hence no net voltage is induced in secondary I3.

The present modulator operates as a so-called side-band modulator. If the sub-carrier frequency is Wc and the picture frequency signal is Wp, then the modulator produces across coil I3 only the sum and difference frequencies Wc-I-Wp and Wc-Wp. Both the picture frequency and sub-carrier frequency signals balance, producing no signals in the output coil I3. This eliminates the use of expensive and complicated filtering in the modulator output. Furthermore, the sense of modulation may easily be controlled, since the sub-carrier. may be balanced for any predetermined value of light on the photoelectric cell. If the balance is made at maximum light, the modulation will be reversed in that black will produce maximum modulated signal output. On the other hand,vif the balance is made on minilmum light the modulation will be in the positive produce a maxidirection in that white will mum modulated signal output.

While one embodiment of the present invention has been shown and described, many modismall, due to the black background. Then it rises to a high value, due to the white bar. Next, it drops to a low value, again due to the black background. Then it rises to an intermediate value, due to the gray bar. Finally, it drops to a low value, due to the black background.

Fig. 3 shows the instantaneous potential of point 5. The greater the current flowing in resistor 42, the greater the positive voltage impressed on grid 20 and hence the greater the plate current drop in plate resistor I5. Thus, the potential of point 5 is high for black, drops to an intermediate, value for gray, and drops to a low value for white. Lines a, b,'c, d and e indicate some of the significant potentials towhich point I I may be adjusted by means of control 39. If the adjustment is made to place point II at a, the resulting modulation of the sub-carrier will be positive and less than percent, as shown at f in Fig. 4. With an adjustment made to b of Fig. 3, the modulation will be positive and 100 percent, as shown at g in Fig. 5. With an intermediate adjustment as shown by c of Fig. 3, no amplitude modulation will result, if the gray height is one-half the white, as shown at h in Fig. 6, although the sub-carrier phase will be reversed when point changes sign with respect to point ll. With an adjustment at d of Fig. 3, negative modulation at V100 percent will result, as shown at m of Fig. 7. Anadjustment .to e of Fig. 3 will also give negative modulation, but of less than 100 percent, as shown at n of Fig. 8. By positive modulation is meant that the subcarrier' amplitude is greatest' on white and by negative modulation is meant that the sub-carrier amplitude is greatest on black. Thus, it will be seen that a simple adjustment positive or negative modulation of any desired degree is obtainable.

What is claimed is:

1. A modulating system comprising four rectiers connected together in a closed loop each with its better conductivity in the same direction around the loop, a center-tapped circuit coupled to a source of signals to be modulated connected across one pair of diagonal points of said loop, a second center-tapped circuit coupled to a load circuit connected across the other pair of diagonal points of said loop, a pair of thermionic vacuum tubes each including at least a cathode, a control grid and a plate, independent load circuits-cone nected to each of said plates, conductive connections between one of said plates and one of said center-taps and the other of said plates and the other of said center-taps, a source of modulating signals connected to one of said grids for modulating the signals applied to said loop, and a source of bias connected to the other of said grids for balancing a predetermined instantaneous value of the second said signal across said loop.

2. In a modulating system the combination of, four rectiiiers connected in a closed loop each with its better conductivity in the same direction, a transformer having a center-tapped secondary connected with its primary across a source of signals to be modulated and its secondary across one diagonal of said loop, a second transformer having a center-tapped primary connected with its primary across the other diagonal of said loop and its secondary across an utilization means, a pair of thermionic vacuum tubes each including at least a cathode, grid and plate. independent load circuits connected between said plates and a source of plate current, conductive connections between one of said plates and one' of said center-taps and between the other of said plates and the other of said center-taps, means for applying signals to one of said grids for modulating the first said signals by means of said rectifiers, and means forapplying an adjustable bias to the other of said grids for controllingthe magnitude and phase of said modulation.

3. In a modulating system the combination of, four rectiers connected in a closed loop each with its better conductivity in the same direction, a transformer having a center-tapped secondary connected with its primary across a source of subcarrier signals and its secondary across one diagonal of said loop, a second transformer having a center-tapped primary connected with its primary across the other diagonal of said loop and its secondary across an utilization means, a pair of thermionic vacuum tubes each including at least a cathode, a grid and a plate, independent load circuits connected between said plates and a source of plate current, conductive connections between one of said plates and one of said centertaps and between the other of said plates and the other of said center-taps, a photo-electric cell connected to one of said grids for modulating said sub-carrier within said loop, and an adjustable bias source effectively connected to the other of said grids for balancing a predetermined component of said modulation within said loop.

4. A modulating system comprising fourrectiiiers connected together in a closed loop, each with its better conductivity in the same direction around the loop, a center-tapped circuit, coupled to a source of signals to be modulated, connected across one pair of diagonal points of said loop, a second center-tapped circuit, coupled to a load circuit, connected across the other pair of di-` agonal points of said loop, an electron stream operated vacuum tube including an output electrode normally maintained at a predetermined operating potential, means for modulating said electron stream, a conductive connection between said output electrode and one of said center-taps for applying the electron stream modulation to one side of said loop, and means for balancing a predetermined portion of said'operating potential across said loop comprising a source of adjustable .bias voltage connected to the second of said center-taps.

5. A modulating system comprising four nonlinear resistors connected together in a closed loop, each with its better conductivity in the same direction around the loop, a center-tapped circuit, coupled to a source of signals to be modulated, connected across one pair of diagonal points of said loop, a second center-tapped circuit, coupled to a load circuit, connected across the other pair of diagonal points of said loop, an electron stream operated vacuum tube including an output electrode normally maintained a'. a predetermined operating potential, means for modulating said electron stream, a conductive connection between said output electrode and one of said center-taps for applying the electron stream modulation to one side of said loopyand means for balancing a predetermined portion of said operating potential across 4said loop comprising a source of adjustable bias voltage connected to the second of said center-taps.

' HUGH .C. RESSLER. 

